1. Field of the Invention
The present invention relates to transmissive targets and X-ray generating apparatuses which are suitably applied to diagnosis application, nondestructive radiography, and the like in fields of medical equipment and industrial equipment.
The present invention particularly relates to a transmission X-ray target including a target layer and a diamond substrate which supports the target layer. The present invention further relates to an X-ray generating tube including the transmission X-ray target, an X-ray generating apparatus including the X-ray generating tube, and an X-ray imaging system including the X-ray generating apparatus.
2. Description of the Related Art
In X-ray generating apparatuses which generate X-rays and which are used for medical diagnosis, there is a demand for improvement of operability of the apparatuses by improving durability and facilitating maintenance so that medical modality which is applicable to home medical care and emergency medical care in cases of disasters and accidents is realized.
Main factors of determining durability of X-ray generating apparatuses include heat resistance of a target serving as an X-ray generating source.
In X-ray generating apparatuses which generate X-ray by irradiating an electron beam to a target, “X-ray generating efficiency” of the target is smaller than 1%, and therefore, most energy supplied to the target is converted into heat. When dissipation of heat generated by the target is not sufficiently performed, an adhesion property of the target is deteriorated due to thermal stress, and accordingly, the heat resistance of the target is restricted.
As a method for improving the “X-ray generating efficiency” of the target, a transmissive target including a target layer of a thin film including heavy metal and a substrate which allows X-ray to be transmitted and which supports the target layer is widely used. Japanese Patent Laid-Open No. 2009-545840 discloses a rotating anode transmissive target having “X-ray generating efficiency” increased by 1.5 times or more relative to a rotating anode reflection target in the related art.
Furthermore, as a method for encouraging external “dissipation of heat” from the target, application of diamond to a substrate which supports a target layer of a lamination target is widely used. Japanese Patent Laid-Open No. 2002-298772 discloses improvement of a heat X-ray property and realization of microfocus by using diamond as a substrate which supports a target layer including tungsten. The diamond is suitable for a support substrate for supporting a transmissive target since the diamond has a high X-ray transmission property in addition to high durability and high thermal conductivity.
However, the diamond has low wettability relative to molten metal and a linear expansion coefficient which mismatches that of solid metal, and accordingly, compatibility with target metal is low. Therefore, to ensure an adhesion property between the target layer and the diamond substrate is an issue to improve reliability of the transmissive target.
Japanese Patent 2002-298772 discloses generation of thermal stress between a target layer and a diamond substrate caused by mismatch of linear expansion coefficients in an X-ray generating tube including a transmissive target and occurrence of peeling and generation of crack in the target layer caused by the thermal stress. According to Japanese Patent Laid-Open No. 2002-298772, since the target layer leans toward the diamond substrate, the target layer is pushed toward the diamond substrate at a time of operation of the X-ray generating tube so that the target layer is prevented from being peeled.
Japanese Patent Laid-Open No. 2012-256444 discloses occurrence of variation of output caused by thermal resistance generated between a diamond substrate and a target layer in an X-ray generating tube including a transmissive target, which is a problem to be solved. According to Japanese Patent Laid-Open No. 2012-256444, since the target layer and a metal carbide layer of metal for forming solid solution are inserted between the target layer and the diamond substrate, an adhesion property between the target layer and the diamond substrate is improved so that the variation of output of X-ray is suppressed.
Even when the transmissive target including the metal carbide layer inserted between the target layer and the diamond substrate is used as the structure disclosed in Japanese Patent Laid-Open No. 2012-256444, variation of output of X-ray may occur since the adhesion property of the target is not sufficiently maintained for a long period of time.